The present invention enables estimation of parameters of a propagating wave field, such as the direction of propagation in 3D space of an acoustic wave from borehole-acoustic data. The estimation technique uses the phase delays between recordings made by the individual receivers related to a plane wave travelling across the receiver array. The estimated plane waves would include the wave field generated directly by the acoustic source, as well as refracted and reflected components of these fields. The technique can be used with overlapping wave fields. This could potentially lead to significant improvements in the quality of formation properties that is estimated from full-waveform data, obtained either from wireline or from while-drilling sonic data.
We also show, using ultrasonic data, that we can use these propagating modes to measure the location and the reflectivity abnormalities outside of multiple casing strings, allowing inference about material properties behind multiple casing strings.
Since the original work by Hornby (1989), with a 12-receiver station tool (EVA, developed by Elf/CGG), Sonic Imaging has been attempted with varying success using a number of different implementations. The root cause for most of the problems with any of these variations is that the well bore is an excellent guide for acoustic waves. This acoustic wave guide would cause energy reflected by the well surroundings to be over-powered by energy trapped in the well bore.
In 2002 and early 2003, a new set of processing algorithms were developed (Haldorsen et al., 2005) using adaptive filters to control the Stoneley waves. These algorithms allowed the use of much shorter source-receiver offsets. A short source-receiver offset is geometrically much more favorable for near-wellbore reflection imaging.
The reflection data are converted into images of the formation using synthetic-aperture processing.
Adaptive filters were also used with the receivers mounted around the perimeter of the tool to determine the azimuth of the acoustic reflectors, in effect creating a 3D image around the borehole. With the new algorithms, Sonic Imaging (with the acronym BARS from Borehole Acoustic Reflection Survey) could be applied to data acquired with the Sonic Scanner in its standard configuration, allowing data produced by any sonic job to be used for imaging.
Ultrasonic pulse-echo techniques were initially developed as a form of acoustic caliper technique (Havira, 1986). Hayman et al. (1994) realizing that the high frequencies, 290-550 kHz, allowed the imaging of the backside of the casing, allowing ultrasonic tools to be used for estimating the thickness of the casing wall. However, the extremely high contrast between the steel of the casing and the fluid or cement behind the casing, would set up standing waves that made it very difficult to make sense of components of the signal that actually had penetrated into the annulus behind the casing.
Zeroug and Froelich (2003) realized that a flexural mode in the casing, traveling along the well, would leak into the annulus, be reflected from features in the annulus, and by recording such waves, that could be able to image the structures behind the casing.
This is the technology behind the Schlumberger Isolation Scanner tool, designed to specifically measure waves refracted along the well, with the purpose to improve the characterization of the annular environment (van Kuijk et al, 2005). The Isolation Scanner has both a pulse-echo transceiver and a combination of transmitter and receivers designed to excite and record flexural waves in the casing. The tool, which rotates at the bottom of the tool, scans the casing at predetermined intervals allowing 360° azimuthal coverage to help identifying channels in the cement and confirming the effectiveness of a cement job for zonal isolation.
The acoustic waves emitted from an acoustic tool in a cased well scattered, reflected and refracted from inhomogeneities in the formation outside the inner casing (see FIG. 1).
According to the present invention acoustic data, generated by a known source and recorded by an array of acoustic receivers mounted on the surface of a cylindrical tool in a wellbore, is separated into propagating plane waves and the propagation parameters for these plane-wave components is estimated individually. This method allows separation of borehole modes from body waves, and where the body waves are used for imaging formation features outside of the well bore. Similarly, the ray parameters of the plane-wave components give information about the direction to a feature in the formation that acts as an acoustic scatter (for body waves), and for the tool eccentricity (for borehole modes).
By using ultrasonic data, these propagating modes is used for measuring the location and the reflectivity of abnormalities outside of multiple casing strings, allowing inference about material properties behind multiple casing strings.